Monoclonal antibodies to human mammary tumor metastases were tested for reactivity to novel and known tumor associated antigens. The monoclonals were used to immunoprecipitate antigens from a radiolabeled breast tumor metastasis extract. Monoclonal antibody B72.3 immunoprecipitated a high molecular weight polypeptide complex of approximately 220,000d. B6.2 and four other antibodies immunoprecipitated a 90,000d polypeptide. The four other antibodies cross-react in RIA for monoclonal B6.2 but differ in their ability to compete with the binding of B6.2. Two antibodies, B1.1 and F5.5, were shown to differentially react with carcinoembryonic antigen. The high molecular weight complex identified by monoclonal B72.3 has been preparatively purified, using molecular sieving and antibody affinity chromatography, without loss of immunoreactivity. Studies are in progress to develop radioimmunoassays with several of the monoclonals described. Radioimmunoassays for monoclonal antibody B72.3 have been established and optimized, with a sensitivity in the nanogram range.